1. Field of the Invention
The present invention relates to an equalizer and an equalizing method and, in particular, relates to an equalizer and an equalizing method, in which equalization of a received signal is carried out using the minimum mean square error method (MMSE: Minimum Mean Square Error) or zero forcing method based on signal processing in the frequency domain.
2. Description of Related Art
Implementing high-speed data transmission in the next-generation wireless communication systems for mobile communications is an important task, but the increased data rates lead to problems associated with inter-symbol interference (multipath interference) due to multipath signals. There are various methods of suppression of such multipath interference, including relatively simple methods based on the use of linear equalizers or proposals involving frequency equalizers, which perform such equalization processing in the frequency domain (for example, see D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency Domain Equalization for Single-Carrier Broadband Wireless Systems”, IEEE Commun. Mag., vol. 40, no. 4, pp. 58-66, April 2002, hereinafter, referred to as “prior art document 1”).
FIG. 8 illustrates an exemplary equalizer configuration that makes use of a frequency equalizer, as described in the prior art document 1. This conventional equalizer comprises receiving antenna 1, filtering part 100, transmission channel estimation part 110, and weight calculation part 6. Receiving antenna 1 receives a digitally modulated single-carrier signal. A received signal sampled using a predetermined number of over-samples is inputted to filtering part 100, which performs filtering. A received signal sampled using a predetermined number of over-samples is inputted to transmission channel estimation part 110, which performs path timing detection, converts transmission channel responses for each path timing to the frequency domain, and calculates transmission channel estimates corresponding to the subcarriers of the received signal in the frequency domain. The frequency-domain channel estimates outputted from transmission channel estimation part 110 are inputted to weight calculation part 6, which calculates the weights of the equalizing filter used in filtering part 100. In the conventional equalizer, all the processing of filtering part 100, transmission channel estimation part 110, and weight calculation part 6 is performed using the same over-sample number.
The filtering part 100 comprises guard interval (GI) removal part 7, serial-to-parallel (S/P) converter 8, FFT (Fast Fourier Transform) part 9, equalizing filter 10, IFFT (Inverse FFT) part 11, and parallel-to-serial (P/S) converter 12. A received signal sampled using an over-sample number of NOS is inputted to GI removal part 7, which removes the portion corresponding to the GI from the received signal. S/P converter 8 performs S/P conversion of the received signal, from which the GI has been removed by GI removal part 7. The received signal converted by S/P converter 8 is inputted to FFT part 9, which converts it to the frequency domain using FFT of NOS×NFFT points (NFFT: FFT block length). The weights calculated by weight calculation part 6 and the received signal converted to the frequency domain by FFT part 9 are inputted to equalizing filter 10, which performs filtering (equalization) of the received signal in the frequency domain. If the received signal converted to the frequency domain in FFT part 9 is designated as X(f) (1≦f≦NOS×NFFT) and the weights calculated by weight calculation part 6 are designated as W(f), then the equalized signal Y(f) filtered by equalizing filter 10 will be given by Y(f)=W(f)X(f). Here, f designates a subcarrier point. The equalized frequency-domain signal outputted by equalizing filter 10 is inputted to IFFT part 11, which converts it to the time domain using IFFT of NOS×NFFT points. P/S converter 12 performs P/S conversion of the signal converted to the time domain and outputs a demodulated signal.
Transmission channel estimation part 110 comprises timing detection part 2, transmission channel response estimation part 3, S/P converter 4, and FFT part 5. The received signal with an over-sample number of NOS is inputted to the timing detection part 2, which uses the pilot signal contained in the received signal to detect the timing of a plurality of paths. Methods used for timing detection include, inter alia, a method for detecting the timing of a plurality of high-level paths based on the results of detection of sliding correlation between the pilot signal contained in the received signal and a known pilot signal sequence. The received signal with an over-sample number of NOS and the path timings detected by timing detection part 2 are inputted to transmission channel response estimation part 3, which uses the pilot signal contained in the received signal to calculate transmission channel estimates at the timing points and to obtain impulse responses. S/P converter 4 performs S/P conversion of the impulse responses of the transmission channels estimated by transmission channel response estimation part 3. The impulse responses of the transmission channel converted by S/P converter 4 are inputted to FFT part 5, which outputs transmission channel estimates converted to the frequency domain using FFT of NOS×NFFT points.
The frequency-domain transmission channel estimates outputted from FFT part 5 are inputted to weight calculation part 6, which calculates the weights of equalizing filter 10. The calculations are based on the minimum mean square error method (MMSE: Minimum Mean Square Error) or the zero forcing method, etc. When using the MMSE method, the weights W(f) of equalizing filter 10 are given by;W(f)=H*(f)/(|H(f)|2+N0)Here, * represents a complex conjugate and N0 noise power.
In a conventional equalizer, timing detection part 2, FFT part 5, weight calculation part 6, FFT part 9, equalizing filter 10, and IFFT part 11 are all operated using an over-sample number of NOS. Accordingly, in order to improve channel estimation accuracy in a conventional equalizer, it is necessary to raise the timing resolution, i.e., the over-sample number NOS, but the problem with increasing the number of over-samples NOS is that it leads to an increase in the processing size of FFT parts 5 and 9 and IFFT part 11, as well as an increase in the computational burden of weight calculation part 6 and equalizing filter 10.
The present invention provides an equalizer and an equalizing method that address such problems and are capable of implementing superior equalization characteristics while suppressing the processing size of the FFT parts in a frequency equalizer performing equalization using frequency domain signal processing.